The invention generally relates to techniques for inducing heart arrhythmias in animal test subjects and in particular to techniques for inducing ventricular arrhythmias of the type that can result in Sudden Cardiac Death.
Sudden Cardiac Death (xe2x80x9cSCDxe2x80x9d) claims about 300,000 lives a year in the United States alone. In most cases, the direct cause of SCD is a ventricular tachycardia (xe2x80x9cVTxe2x80x9d) which triggers a ventricular fibrillation (xe2x80x9cVFxe2x80x9d). VT and VF are different types of ventricular arrhythmias. VT is an abnormally fast ventricular heart rhythm which is, by itself, typically not fatal. VF is a chaotic ventricular heart rhythm which produces little or no net blood flow from the heart, such that there is little or no net blood flow to the brain and other organs. VF, if not terminated, results in death. In most cases of SCD, the victim has a previous myocardial infarction (xe2x80x9cMIxe2x80x9d), i.e. the patient had a previous heart attack caused by blockage of a portion of the coronary artery which supplies blood to the heart muscle. As a result of the blockage, a portion of the heart muscle does not receive blood and therefore becomes scarred and diseased. The scarred and diseased portion of the heart is referred to as the MI. For several days immediately subsequent to the occurrence of an MI, numerous episodes of VTxe2x80x94referred to as phase one episodes-typically occur. Eventually, the phase one VT episodes largely disappear. Several days or weeks later, though, additional episodes of VTxe2x80x94referred to as phase two episodes-typically begin to occur. It is the phase two episodes of VT that often transition to VF resulting in SCD of the patient
Accordingly, it would be highly desirable to develop techniques for preventing VT from occurring, particularly phase two VT in a patients having an MI and, if VT does occur, for preventing the VT from transitioning to a VF. One technique employed in an attempt to prevent VT from occurring is overdrive pacing of the heart. With overdrive pacing, the heart is paced at a rate higher than its intrinsic pacing rate. If VT nevertheless occurs, one or more electrical cardioversion pulses are typically applied to the heart in an attempt to terminate the VT so that the VT does not transition to VF. If VF nevertheless occurs, one or more stronger electrical defibrillation pulses are typically applied to the heart in an attempt to terminate the VF thereby preventing SCD. Hence, for patients that have an MI, particularly a significant one, an implantable cardioverter-defibrillator (ICD) is often implanted into the patient. The ICD includes components for overdrive pacing the heart and for detecting VT or VF and for administering the appropriate therapy. However, the need to frequently overdrive pace the heart and to deliver cardioversion or defibrillation pulses can quickly deplete the battery power of the ICD requiring frequent replacement. Also, the therapies administered by the ICD, particularly the application of cardioversion pulses, may be extremely painful to the patient. Moreover, in some cases, the conventional therapy provided by the ICD is not sufficient to prevent or terminate VF and, accordingly, the patient succumbs to SCD.
Hence, it would also be highly desirable to develop new and more effective techniques for predicting and preventing VT from occurring, particularly prevention techniques that do not require frequent overdrive pacing. For example, it would be desirable to provide a technique for predicting when an episode of VT is imminent such that overdrive pacing, or other therapies, need only be applied in circumstances when VT is likely to occur. A reliable prediction technique could greatly reduce need for overdrive pacing, thereby substantially extending the lifetime of the power supply of the ICD and eliminating the need for frequent surgical replacement of the battery supply. Most importantly, with reliable VT prediction, techniques for preventing the occurrence of VT may prove to be more reliable, thus reducing the number of episodes of VT and consequently reducing the need for cardioversion therapy and reducing the risk that the VT may transition to VF if cardioversion fails.
It would also be highly desirable to develop new and more effective techniques for predicting and preventing VT from transitioning to VF, preferably without the need for cardioversion pulses. For example, it would be desirable to provide a technique for predicting when an episode of VT is likely to transition to VF such that cardioversion therapy need only be applied when there is a significant risk that the transition will occur. A reliable prediction technique could greatly reduce the need for cardioversion therapy which, as noted, can be extremely painful to the patient. Also the lifetime of the power supply of the ICD is further extended thus reducing the need for surgical replacement procedures. Most importantly, with reliable VF prediction, techniques for preventing the occurrence of VF may prove to be more reliable, thus reducing the number of episodes of VF and consequently reducing the need for defibrillation therapy and reducing the risk of SCD if defibrillation fails.
The new and more effective prediction or prevention techniques could take the form of new detection or analysis techniques performed by the ICD, or new therapies administered by the ICD. Alternatively, the techniques could take the form of therapies not requiring implantation of an ICD, such as application of new drug therapies and the like. Considerable research is ongoing in these areas.
A significant problem, however, is that there is currently no effective technique for inducing ventricular arrhythmias in test animals, particularly ventricular arrhythmias of the type resulting in SCD, in such a manner that permits effective testing of techniques intended to predict and prevent the arrhythmias to thereby prevent SCD. More specifically, it would be highly desirable to provide a technique for inducing VT or VF within test animals which would permit researchers to analyze conditions within the heart leading up to VT or VF for the purposes of identifying conditions which might serve to predict VT or VF. A technique for reliably inducing VT or VF within test animals, would also assist researchers in developing and testing new techniques for preventing VT or VF from occurring and for evaluating the efficacy of such techniques. As one example, therapies for the purposes of preventing VT from transitioning to VF without requiring a cardioversion pulse could be tested by implanting an ICD within an animal test subject, then inducing VT to thereby verify that the ICD reliably prevents the VT from transitioning to VF and thereby prevents or at least reduces the risk of SCD. Such tests would also prove helpful in gaining government approval for marketing ICD""s or other implantable medical devices intended to prevent SCD.
More specifically, there appear to be no effective techniques for inducing VT or VF within test animals which effectively reproduce the conditions with which SCD typically occurs in human patients, i.e., phase two VT to VF transitions within patients subject to a previous MI. One conventional technique, for example, requires applying electrical stimulation pulses to the heart to artificially induce VT. However, if VT is artificially induced by application of electrical pulses, any characteristics of naturally occurring VT which could act as a reliable predictor of a subsequent transition to VF are difficult, and perhaps impossible, to detect. Likewise, techniques intended to prevent the VT to VF transition are difficult to test in circumstances where artificial electrical pulses are simultaneously being applied to the heart. Rather, reliable testing of techniques for predicting and preventing a VT to VF transition are best performed if the VT to VF transition arises in substantially the same manner in which it would naturally arise within patients, particularly those patients already subject to an MI.
Another conventional technique for artificially inducing SCD within test animals, is to create an acute ischemia sufficient to induce VT. An acute ischemia is a complete or nearly complete loss of blood supply to the heart typically resulting in a massive MI and death of the test subject. Techniques intended to predict or prevent a VT or a VF within patients having a mild MI may not be effectively tested in circumstances where a much more severe ischemia is imposed upon the heart of the test animal. Furthermore, many SCD episodes are not associated with any evidence of ischemia. Therefore, induction of SCD by ischemia does not necessarily simulate what actually happens in patients.
Thus, it would be highly desirable to provide a technique for inducing arrhythmias in animal test subjects, which induces the arrhythmias in a manner similar to which arrhythmias normally occur within human patients. In particular, it would be highly desirable to provide a technique for inducing ventricular arrhythmias of the type leading to SCD in patients having a previous MI. It is to these ends that the invention is primarily directed.
In accordance with one aspect of the invention, a method is provided for increasing the likelihood of the occurrence of an arrhythmia in a heart, particularly a ventricular arrhythmia of the type leading to SCD. The method includes the steps of creating an atrioventricular (AV) block in the heart of an animal test subject, inducing an MI in the heart of the test subject, and then stimulating myocardial hyperinnervation in the test subject.
In an exemplary embodiment of the method, the AV block is created by ablating the AV node of the heart using an ablation catheter. The MI is induced by ligating the left anterior descending portion of the coronary artery. Myocardial hyperinnervation is stimulated by application of nerve growth factor (xe2x80x9cNGFxe2x80x9d) or other neurotrophic vectors to the left stellate ganglion. Alternatively, electrical stimulation signals are applied to the left stellate ganglion. The test subject may be, for example, an adult canine.
By creating an AV block and an MI within the heart of an adult canine test subject, then stimulating nerve growth within the left stellate ganglion of the subject using NGF, it has been found that there is a significant increase in the likelihood of SCD arising from phase two ventricular arrhythmias. It is believed that the SCD of the test subject arises in a manner very similar to circumstances wherein SCD occurs in human patients subject to a previous MI. Thus, the method permits SCD to be induced within test animals in a manner facilitating the collection of data pertinent to conditions within the heart arising prior to SCD and for testing techniques intended to prevent SCD, particularly techniques intended to prevent phase two VT and VF within patients subject to a previous MI. Hence, other aspects of the invention are directed to methods for collecting data pertinent to predictors of arrhythmias, particularly phase two VT and VF in patients subject to a previous MI, to facilitate development of techniques for predicting and preventing the arrhythmias. Still other aspects of the invention are directed to methods for testing techniques intended to predict or prevent the onset of arrhythmias, again particularly phase two VT and VF in patients subject to a previous MI. An animal model system for artifically inducing a heart arrhythmia is also provided. Still other objects, advantages and features of the invention will be apparent from the detailed descriptions which follow in combination with the attached drawings.